arXiv subjects

Luca Haag

Publications and source records attributed to Luca Haag.

3 recordsLinked to original sources

arXiv2025

Optical signatures of bulk g-wave altermagnetism in MnTe

For planar d-wave altermagnets, it has been shown that a spin polarization can be induced in a controlled fashion by ultrashort-pulse excitation, even though the material is magnetically compensated. Here, we theoretically analyze the response of the prototypical bulk $g$-wave altermagnet $\alpha$-MnTe to linearly polarized ultrashort pulses. We demonstrate how the electronic spin response in $\alpha$-MnTe exhibits different symmetry characteristics by calculating the excited electron distributions based on ab initio band structure data. These characteristics depend not only on the nodal planes of the bulk g-wave altermagnet, but also on the excitation pulse. We present a simple procedure to analyze the excited-state characteristics via two-dimensional cuts through the three-dimensional Brillouin zone, which can be used as guidance to present-day magneto-optical techniques.

cond-mat.mtrl-sci↗

arXiv2024

Ultrafast electron dynamics in altermagnetic materials

Altermagnets constitute a new class of magnetic materials that combine properties previously thought to be exclusive to either antiferromagnets or ferromagnets, and have unique properties of their own. In particular, a combination of symmetries connecting magnetic sublattices gives rise to a band spin splitting exhibiting unconventional d, g, or i-wave character. Their unique electronic properties have already led to new spin-dependent transport effects. Here, we consider their spin and charge dynamics on ultrafast timescales. We use a minimal tight binding model that captures the main features of the altermagnetic candidate material KRu$_4$O$_8$. In the framework of this model, we compute the spin-dependent electronic scattering dynamics after ultrashort-pulse excitation and show through these microscopic calculations how electron-electron and electron-phonon scattering processes redistribute optically excited carriers in a 2D slice of the Brillouin zone. We find that the optically excited spin polarization is long lived (~1ps) compared to the electron-electron momentum scattering lifetime of roughly 10fs. This contrasts remarkably with the much shorter spin lifetimes observed in typical ultrafast electronic spin dynamics in conventional ferromagnets and antiferromagnets, making these pulse-driven spin excitation experiments a key probe of altermagnetism.

cond-mat.mtrl-sci↗

arXiv2024

All optical excitation of spin polarization in d-wave altermagnets

The recently discovered altermagnets exhibit collinear magnetic order with zero net magnetization but with unconventional spin-polarized d/g/i-wave band structures, expanding the known paradigms of ferromagnets and antiferromagnets. In addition to novel current-driven electronic transport effects, the unconventional time-reversal symmetry breaking in these systems also makes it possible to obtain a spin response to \emph{linearly polarized} fields in the optical frequency domain. We show through ab-initio calculations of the prototypical d-wave altermagnet RuO$_2$, with a symmetry combining twofold spin rotation with fourfold lattice rotation, $[C_2\|C_{4z}]$, that there is an optical analogue of a spin splitter effect, as the coupling to a linearly polarized exciting laser field makes the d-wave character of the altermagnet directly visible. By magneto-optical measurements on RuO$_2$ films of different thicknesses ranging from $2$ to $8\,$nanometers, we demonstrate the predicted connection of the linear polarization of an ultrashort pump pulse to the sign and magnitude of the optically excited electronic spin polarization in the ultrathin RuO$_2$ films. The possibility of exciting and controlling an electronic spin polarization by linearly polarized optical pulses in a compensated system is a unique consequence of the altermagnetic material properties. Our experimental results therefore establish an optical pump-probe based protocol for detection of altermagnetic characteristics in ultrathin RuO$_2$ films, but our all-optical approach should apply more generally to materials in this altermagnetic symmetry class.

cond-mat.mtrl-sci↗